Dynamic and pluggable flaky test handling.

[artem-zinnatullin]

Composer should be able to handle test flakiness dynamically and provide several default strategies (retry n times, put to the end of queue again, etc).

User should be able to customize behavior with parameters and/or plugin.

[emartynov]

We use barista for that but would be nice to have it in composer

[Sloy]

We use Barista for that too (actually, we built Barista for that 😄). But I think it's a good idea to handle this from the test runner itself. Our Barista rules are kind of hacky, they might break the test if not used properly.